1. Field of the Invention
The present invention relates to a dewatering instrument for a paper machine twin-wire former, which is of a type of conducting a dewatering operation through the use of a dewatering inhibiting blade.
2. Description of the Related Art
So far, a twin-wire former has been known as a paper layer forming apparatus for a paper machine. This twin-wire former includes two wires each shaped into a loop configuration, where the moisture is removed from a paper raw material liquid by various dewatering instruments while the paper raw material liquid travels in a state of being put between the two wires, thereby gradually forming a fiber mat which in turn, develops into a web.
FIG. 5 illustratively shows a structure of one example of twin-wire former, and referring to this figure, a description will be made hereinbelow of a paper layer forming apparatus for the twin-wire former.
As shown in FIG. 5, a paper raw material liquid 2 is accommodated within a head box 1, and the paper raw material liquid 2 is spouted out from the interior of the head box 1 toward a gap (paper gap) 15 (see FIGS. 6 and 7) defined by the upper and lower wires 3, 4.
The upper wire (top wire) 3 is guided by a forming roll 11 and guide rolls 3A to 13E, while the lower wire (bottom wire) 4 is guided by a breast roll 12, guide rolls 14A to 14D and others, with the gap 15 being defined between these upper and lower wires 3, 4. The paper raw material liquid 2 develops into a paper layer while traveling within the gap 15.
More specifically, the upper and lower wires 3, 4 rotate to shift in a given direction (in FIG. 5, in the right-hand direction), and the paper raw material liquid 2 moves within the gap 15 at a speed substantially equal to that of the wires 3, 4. The gap 15 is gradually reduced toward the downstream side in the traveling direction, and the loop of each of the wires 3, 4 on the upstream side of the gap 15 is shaped into a curved configuration whose radius of curvature is R. On the upstream side of the gap 15, there are provided a first dewatering instrument 5, a second dewatering instrument 6 and a third dewatering instrument 7 arranged in order. Further, on the downstream side of these first to third dewatering instruments 5 to 7, located are a suction couch roll 8 and a transfer box 9.
The first dewatering instrument 5 is placed within the bottom wire 4 loop with a radius of curvature of R. As shown in FIG. 6, in the first dewatering instrument 5, a plurality of dewatering blades 20 are respectively connected to fitting sections 30A of a base 30 in a state of being spaced from each other, and while the paper raw material liquid 2 travels along the gap 15 with a curved configuration approximate to the radius of curvature assuming R, owing to the dewatering pressure occurring by the crimps or bending of the top wire 3 and the bottom wire 4 on the respective dewatering blades 20, the dewatering takes place toward both the sides (the upper and lower wires 3, 4 sides) (see arrows indicated with the terms "white water"), thus gradually developing into a fiber mat within the gap 15.
Furthermore, the second dewatering instrument 6 is located within the loop of the top wire 3 with the radius of curvature of R, and is, as shown in FIG. 7, equipped with a plurality of dewatering inhibiting blades 21 on fitting sections 33 of a base 31, and an inclined surface 24 and a plane section 23 are formed on the wire side in each of the dewatering inhibiting blades 21. The plane section 23 comes into contact with the top wire 3 to support it. An inclined surface 24 is located on the upstream side of the plane section 23 in the wire traveling direction, and is disposed to gradually separate from the top wire 3 toward the upstream side in the wire traveling direction.
In addition, a wedge-shaped space 25 is defined between each of the inclined surface 24 and the top wire 3. The inclination angle .theta. of the inclined surface 24 is referred to as a wedge-angle. In this second dewatering instrument 6, the dewatering toward the top wire 3 side is suppressed by the dewatering inhibiting blades 21 while the dewatering is allowed to only the bottom wire 4 side, thereby gradually forming a web.
The third dewatering instrument 7 is called a suction box, and is positioned within the loop of the bottom wire 4. The dewatering based upon vacuum is made in this third dewatering, instrument 7 and the suction couch roll 8, and a web produced through the transfer box 9 is surely delivered onto the bottom wire 4 and then transferred through a non-shown suction pickup roll to the next press part.
Meanwhile, in the case of using the prior dewatering inhibiting blades 21 (see FIG. 7) as a dewatering instrument, since the dewatering inhibiting blades 21 have the same configurations, as shown in FIG. 8, the dewatering pressures occurring between the two wires 3, 4 substantially assume the same level throughout the entire forming position (position in the direction of flow of the paper raw material liquid). FIG. 8 shows measured data, and although the dewatering pressure level slightly varies due to the fitting accuracy of the dewatering blades, it is considered that such a difference substantially makes the same level.
In accordance with the peak value of the dewatering pressure, a shear force takes place due to a relative speed difference in the wire or the paper raw material liquid between the fiber mat layers, and this shear force takes action to uniformly disperse the fibers.
However, since the paper raw material liquid is dewatered as it goes toward the further downstream side in the flow direction, the fiber concentration increases and the mobility (the easiness of movement of the fibers) deteriorates, so that a serious problem arises in that the dispersion performance or ability of the fibers decreases if the same shear force is applied thereto.
As a countermeasure against this problem, there may be taken a technique in which, as shown in FIG. 9, the dewatering inhibiting blades 21 are disposed intermittently and the dewatering blades 22 are disposed at a position being in an opposed relation through the wires 3, 4 to the dewatering inhibiting blades 21 to be movable toward the bottom wire 4 to adjust the pressing forces by the dewatering blades 22 so that the dewatering pressure peak value is adjustable.
That is, a fitting member 34 for the dewatering blade 22 is disposed to be movable with respect to the base 32, and an air-pressure giving member 35 is put between the fitting member 34 and the base 32 to bias the fitting member 34 and the dewatering blade 22 toward the bottom wire 4 side by the air pressure. Through this air-pressure adjustment, the dewatering pressure peak value is adjustable to maintain the fiber dispersion performance.
There is a problem which arises with such a means, however, in that, although the fiber dispersion is improvable, since the dewatering of the paper raw material is made from both the top wire 3 side and the bottom wire 4 side, the short fibers at the central portion in the direction of the thickness of the paper layer move toward the external layer sections to decrease in the intermediate layer section, and therefore, the inter-fiber coupling strength in the intermediate layer section becomes weak to cause the strength of the formed paper in its thickness directions to decrease.